Artificial impedance surface antennas are realized by launching a surface wave across an artificial impedance surface, whose impedance is spatially modulated across the impedance surface. The basic principle of artificial impedance surface antenna operation is to use the grid momentum of the modulated impedance surface to match the wave vectors of an excited surface-wave front to a desired plane wave.
Impedance surfaces can support surface wave modes, which are TM, TE or hybrid. Hybrid modes are supported on tensor impedance surfaces (not scalar surfaces), and are a combination of TM and TE modes. These hybrid modes have previously been classified as two types, “TM-like” and “TE-like” due to their similarity with the pure TM and TE modes. TM modes have electric field polarized in the propagating direction and normal to the surface, and magnetic field in the transverse direction. TE modes conversely have magnetic field in the propagating direction and normal directions and electric field in the transverse direction.
In “Artificial Impedance Surface Antenna Design And Simulation,” by D. Gregoire and J. Colburn, in Proceedings of the 2010 Antenna Applications Symposium, Vol II, 288 (2010), the authors report the development of a fast approximate method for simulation of artificial impedance surface antennas that can rapidly compute radiation patterns for flat and curved artificial impedance surfaces. As part of the development process, the authors noted that while TM-mode artificial impedance surface antennas are limited in their angular range, TE-mode artificial impedance surface antennas can radiate very efficiently at high angles of elevation because each current element is perpendicular to the surface-wave propagation, and there is no angular dependence polarization. This identifies at least one motivation for a designer to have a means to convert TE-mode and TM-mode polarizations.
Designers of electrically-scanned antennas, electromagnetic scattering, reflect arrays, waveguides and other electromagnetic devices desire the flexibility to switch between polarizations within a single design. A typical challenge for such designers has been the integration of antennas onto complex metallic shapes while retaining the desired radiation characteristics.
In “Holographic Artificial Impedance Surfaces for Conformal Antennas”, by D. Sievenpiper, J. Colburn, B. Fong, J. Ottusch, and J. Visher, in 29th Antennas Applications Symposium, (2005), an artificial impedance surface consisting of a lattice of sub-wavelength metal patches on a grounded dielectric substrate is disclosed. The effective surface impedance of the disclosed structure depends on the size of the patches, and can be varied as a function of position. Using holography consisting of patterns of metal strips, the surface impedance is designed to generate any desired radiation pattern from currents in the surface. However, no reference is made to polarization and no disclosure of polarization conversion is disclosed.
Previous art have disclosed TM, TE, or TM-like surfaces. In “A Steerable Leaky-Wave Antenna Using A Tunable Impedance Ground Plane,” by D. Sievenpiper, J. Schaffner, J. Lee, and S. Livingston, in IEEE Antennas and Wireless Propagation Letters, Vol. 1, No. 1, 179, (2002), a prior art steerable leaky-wave antenna is disclosed, wherein a horizontally polarized antenna couples energy into leaky transverse electric waves on a tunable textured ground plane. The tuned resonance frequency of the surface, shifts the band structure in frequency changing the tangential wave vector of the leaky waves for a fixed frequency and steering the elevation angle of the resulting radiated beam. While TM and TE modes are discussed, this prior art does not suggest or disclose a way to convert polarizations of an incident wave.
In “Simple and Accurate Analytical Model of Planar Grids and High-Impedance Surfaces Comprising Metal Strips or Patches,” by O. Luukkonen, C. Simovski, G. Granet, G. Goussetis, D. Lioubtchenko, A. Raisanen, and S. Tretyakov, in IEEE Antennas and Propagation, Vol. 56, No. 6, 1624, (2008), the authors suggest an analytical model capable of predicting the plane-wave response of artificial surfaces for large angles of incidence, including of TE- and TM-polarized waves. While the authors discuss the conduct of the waves on the artificial surfaces, they do not discuss the conversion of the polarization modes or employ mechanisms to alter the polarized waves on the artificial surfaces.
In other prior art, “Adaptive Artificial Impedance Surface Conformal Antennas,” by J. Colburn, A. Lai, D. Sievenpiper, A. Bekaryan, B. Fong, J. Ottusch, and P. Tulythan, in Antennas and Propagation Society International Symposium, 1, (2009), discloses an approach to controlling the radiation from surface waves propagating on an adaptable impedance surface wherein varactors are inserted between small metal pads. By varying the voltage bias between the metal pads, different impedance patterns can be created allowing the antenna to be sufficiently agile to make conformal antennas that are adaptable both in frequency and radiation pattern. In addition microwave holograms are created using the interference pattern between the expected bound TM surface wave and the desired outgoing plane wave. No polarization conversion is suggested or disclosed.
In B. Fong, J. Colburn, J. Ottusch, J. Visher, and D. Sievenpiper, “Scalar and Tensor Holographic Artificial Impedance Surfaces,” IEEE Transactions On Antennas And Propagation, Vol. 58, No. 10, 3212 (2010), this prior art discloses TM and TM-like holographic antennas. In FIG. 1, a tensor artificial impedance surface, implementing a slice through square metal patches having a variable angle and gap width, are used to design conformal antennas to scatter a given surface wave into a desired far-field radiation pattern and provide polarization control. This prior art discloses means to design and build a surface to generate a circularly polarized plane wave from a linearly polarized source. However, this prior art does not disclose a structure or method to convert the polarization of surface waves between pure TE and TM polarizations.
In D. Gregoire and J. Colburn, “Artificial Impedance Surface Antennas,” Proceedings of the 2011 Antenna Applications Symposium, 460 (2011), the prior art identifies structures that support surface waves that are polarized in either transverse electric (TE) or transverse magnetic (TM) modes. In FIG. 2, the authors report a square patch, with an angled slice through it, has been used to form tensor surface wave structures, in order to realize holographic leaky-wave antennas with circularly polarized radiation.
In A. Patel and A Grbic, “A Printed Leaky-Wave Antenna Based on a Sinusoidally-Modulated Reactance Surface,” IEEE Transactions on Antennas and Propagation, Vol. 59, No. 6, 2087, (2011), this prior art provides for designing a reactance surface that generates directive radiation at a desired off-broadside angle for a fixed frequency. In particular a printed leaky wave, TM polarized antenna with a modulated reactance surface is designed using an array of metallic strips, with the gaps between metallic strips mapped to a desired surface impedance, over a grounded dielectric substrate. Neither use of TE polarization for this prior art reactance surface design nor conversion between TE and TM polarization are disclosed.
Tensor impedance surfaces have a tensor relationship between the electric and magnetic fields on the surface. This relationship is defined by the 2×2 surface impedance tensor:
            Z      surf        =          [                                                  Z              xx                                                          Z              xy                                                                          Z              yx                                                          Z              yy                                          ]        ,          ⁢      E    =                            Z          surf                ⁡                  (                                    z              ^                        ×            H                    )                    .      
Impedance surfaces are most commonly created by periodically patterning sub-wavelength metallic inclusions into a dielectric. The periodicity of the inclusions are generally on the order of λ/10. For larger periods, surfaces support multiple surface wave modes which can interfere. The TM-like mode breaks down at the cutoff of the lowest TE mode. Above this cutoff frequency the TM-like mode can no longer be used for certain incidence angles. In prior art designs, the antenna is operated below this TE cutoff frequency. In the present invention it is shown that a polarization converter can be created by operating above the TE cutoff frequency. The mode is not a TM-like mode but instead a true hybrid TM-TE mode. Hybrid TM-TE modes are not correctly modeled by a single tensor impedance boundary. Instead, a grounded dielectric with a tensor impedance sheet on the top layer is used to model the structure. Extraction methods for capacitive impedance sheets on grounded dielectric substrates have been developed.
The main advantage of the present invention is that it allows a designer to switch between polarizations within a single design. It can also alter (either increase or decrease) coupling between antennas or objects on a surface.